1. Field of the Invention
The present invention generally relates to an image recording device and, more particularly, to an electrophotographic image recording device including a transfer unit having a transfer roller and opposed photosensitive body such that a sheet of paper (recording paper) is nipped between the transfer roller and photosensitive body and a toner image on the photosensitive body is transferred onto the recording paper by the transfer roller upon application of electricity to the transfer roller.
2. Description of the Related Art
Generally, electrophotographic image recording devices include an electric charger for uniformly charging a surface of a photosensitive drum, an exposing unit for illuminating the photosensitive drum surface to form an electrostatic latent image, a developing unit for adhering a toner on the electrostatic latent image to form a toner image and a transfer unit for transferring the toner image onto the recording sheet. Conventional transfer units are designed as a kind of corona discharger and corona discharge from the transfer unit applies a high voltage to a transfer roller located to oppose the photosensitive drum when the toner image is transferred. A voltage difference between the photosensitive drum and transfer unit causes the toner image on the photosensitive drum to transfer to the recording sheet.
Because the corona discharge accompanies generation of ozone, image recording devices which generate less ozone are desired in view of environmental conservation. A toner image transfer technique which does not depend upon the corona discharge is ideal. In order to meet such a need, there is a proposal which constructs a transfer unit using a roller made from an electrically conductive urethane foam or the like and a prescribed high voltage is applied to the transfer roller to establish a potential difference between the photosensitive drum and transfer roller which are opposed to each other over the recording sheet interposed therebetween.
However, the transfer roller made from urethane foam changes its electric resistance with environmental conditions such as temperature and humidity. Therefore, even if a predetermined voltage which has been determined beforehand under a particular condition is applied to the transfer roller such that the transfer roller would have a desired voltage (i.e., an optimum voltage for transferring of the toner image), the value of an actually optimum voltage may have already changed and may not be the same as the predetermined voltage due to change of the environmental conditions. If the transfer roller is not set to the optimum voltage during the transfer operation, the toner image will not sufficiently adhere onto the recording sheet, which degrades quality of image after transfer.
Another conventional image recording device is disclosed in Japanese Patent Application, Laid-Open Publication No. 8-123222. This image recording device adjusts the application voltage depending upon various conditions, but the resistance of the transfer roller and environmental temperature are not among these conditions.
Still another conventional image recording device is disclosed in Japanese Patent Application, Laid-Open Publication No. 4-275583. This image recording device determines a voltage Vt' to be applied to a transfer member by the equation of Vt'=aVt+b where Vt is the voltage of the transfer member of when no image transfer operation is conducted, and "a" and "b" are compensation coefficients. The voltage Vt is compared with a reference value and the coefficients "a" and "b" are changed based on a difference between the voltage Vt and the reference value such that the application voltage Vt' is adjusted according to changing conditions.